This invention relates in general to mass transfer and exchange columns, and, more particularly, to a vapor-liquid contact tray and downcomer assembly employed in such columns. The invention also relates to a process in which the tray and downcomer assembly is utilized to improve the contact between vapor and liquid streams flowing through the column.
Horizontally disposed trays are used in many types of mass transfer or exchange columns to facilitate the contact between upwardly flowing vapor streams and downwardly flowing liquid streams. These vapor-liquid contact trays are formed from a solid sheet-like material and contain a plurality of apertures which allow vapor to flow upwardly through the tray for interaction with liquid flowing across the top surface of the tray. In trays known as sieve trays, the apertures are sized small enough so that during operation of the column the pressure of the vapor passing upwardly through the apertures restricts or prevents liquid from passing downwardly through the apertures. In other types of trays such as valve trays, structural elements such as valves, bubble caps, and tunnel caps can be provided about the apertures to seal against the downward passage of liquid.
Downcomers are conventionally provided in combination with the vapor-liquid contact trays to provide a passage through which liquid is removed from one tray and directed to an underlying tray. In single pass trays, the downcomers are provided at opposite ends of vertically adjacent trays so the liquid must flow completely across one tray before it enters the downcomer for passage to the next lower tray. The liquid on the lower tray then flows in the opposite direction across the tray and enters another downcomer. This back-and-forth flow pattern is repeated as the liquid descends through the portion of the column containing the vapor-liquid contact trays. In double pass trays, the liquid is split into two streams which travel in opposite directions on each tray. A center downcomer is provided on alternate trays while two end downcomers are placed at opposite ends of vertically adjacent trays to provide the double pass flow pattern. Multiple pass trays are also utilized and are constructed in a similar manner using multiple downcomers.
A weir is also used on vapor-liquid contact trays to cause liquid to accumulate on the top surface of the tray for enhanced interaction between the accumulated liquid and the vapor bubbling through the apertures in the tray deck. The vapor and liquid interaction on the tray desirably causes a froth to build up on the tray. Because the liquid phase remains substantially continuous in the froth, the vapor and liquid interaction continues in the froth and results in greater mass transfer efficiencies.
The area of the tray deck which contains the apertures in conventional vapor-liquid contact trays is referred to as the "active area" of the tray because the vapor-liquid interaction occurs above the apertures in the tray. In general, the liquid and vapor handling capacity of the tray is limited by the available active area of the tray as well as the area of the downcomer. If the amount of descending liquid or ascending vapor exceeds the tray capacity, flooding of the tray will occur as either the entrained liquid is unable to adequately disengage from the associated vapor stream or the vapor is unable to disengage from the liquid stream.
The active area on many conventional trays does not include the area immediately below the outlet of the downcomer which is associated with the overlying tray. This area of the tray below the downcomer outlet is referred to as the downcomer receiving area and is typically a solid plate which receives the vertically flowing discharge from the downcomer and redirects it horizontally to flow across the tray.
Because greater tray capacities can be obtained by increasing the active area on conventional trays, attention has been focused on expanding the active area of the tray into that portion of the tray underlying the downcomer outlet. However, it is impracticable to simply place apertures in the downcomer receiving area on the tray because the liquid exiting vertically downward from the downcomer could be forced through such apertures. In addition, the presence of apertures in the receiving area could cause vapor to flow upwardly from the receiving area and enter the downcomer where it would interfere with the downward flow of liquid in the downcomer.
One known method of increasing the active area on trays involves decreasing the cross-sectional area of the downcomer outlet by sloping the normally vertical wall of the downcomer to cause a constricted discharge outlet. This reduced area of the downcomer outlet allows more apertures to be placed in the tray deck without being located directly under the downcomer outlet and thus increases the active area of the tray. It is also known, as disclosed in U.S. Pat. No. 5,049,319, to place a seal pan between the tray deck and the downcomer outlet. This allows apertures to be placed in that portion of the tray underlying the seal pan to increase the active area of the tray. In U.S. Pat. Nos. 4,956,127 and 5,164,125, a perforated raised deck is provided beneath the downcomer and the outlet of the downcomer is constructed to form a dynamic seal against entry of ascending vapor. The raised deck is referred to as a raised active inlet area and is said to cause vapor injection into the liquid being discharged from the downcomer, thereby enhancing mass transfer.
Although the capacities obtainable with the improved vapor-liquid contact trays referenced above are notable, the complicated nature of those tray designs may make them too costly for use in many types of processes. In addition, although such trays can be designed to operate efficiently within a limited range of liquid and vapor flow rates, the efficiency of the trays are substantially compromised when the liquid or vapor flow rate falls outside the designed range. A need has thus developed for a less complicated tray design which provides the desired capacity and the desired efficiency over a greater range of operating flow rates.